Pressure-Sensitive Adhesive Art PA0 Copolymer Art
Normally tacky pressure-sensitive adhesive (hereinafter referred to by the abbreviation "PSA") compositions suitable, for example, for use in adhesive tapes must have an art-recognized (1952 Fall Symposium, Division of Paint, Varnish and Plastics Chemistry, American Chemical Society) four-fold balance of adhesion, cohesion, stretchiness and elasticity. PSA coated tapes have been produced and sold for at least a half century.
The early PSA tapes relied upon natural rubber for the elastomeric base and wood rosins as tackifiers to provide adhesive compositions with the requisite four-fold balance of properties. While tackified natural rubber provided a PSA composition which was of commercial significance, improvements in such compositions were sought because of the expanded expectation level of performance of PSA compositions. Various improved PSA compositions were thus developed.
Ionic polymerization produced block copolymer elastomers such as linear AB and ABA block copolymers which were likely candidates for the elastomer base in the PSA compositions and many were incorporated into such compositions to produce adhesives having high performance characteristics. For example, Harlan (U.S. Pat. No. 3,239,478) produced PSA compositions based on ABA block copolymer, tackifier resin and extender oil, recognizing that improved tack and cohesive strength could be obtained despite a heavy loading of extender oil. Miller (U.S. Pat. No. 3,519,585) produced an improved PSA composition having high peel strength, creep resistance and tack by blending AB and ABA block copolymers with a tackifier resin.
Other elastomer candidates for preparing PSA compositions include radial teleblock copolymers and multiarm star block copolymers. The various polymer structures described by the terms "branched", "radial" and "star" are not the same. "Branched" is a generic term indicating a nonlinear structure which may contain various polymeric subunits appended to various places on a main polymer chain or backbone. Such structures are typically complex in nature and may be derived by free radical or cationic polymerization. The term "radial" generally refers to branched polymer structures obtained by linking individual polymeric segments to yield a mixture of polymers having four or fewer arms joined centrally. The term "star" describes the structure of a multiarm polymer with copolymer arms which are joined together at a nucleus formed of a linking group which is virtually a point relative to the overall size of the remainder of the polymer structure. Non-terminating coupling agents, those in which the polymerizing anionic structure is retained, are generally preferred as linking agents for "star" structures.
While several references disclose preparing adhesive compositions or PSA compositions employing radial teleblock copolymers and multiarm star block copolymers, none have recognized that novel anionically-prepared copolymers containing organometallic-substituted styrene may be used to prepare PSA compositions nor that such compositions exhibit unusual melt viscosity characteristics as well as excellent adhesive properties. For example, St. Clair (U.S. Pat. No. 4,444,953) describes asymmetric star block polymer prepared by terminally linking together a mixture of styrene-isoprene AB block polymers and isoprene homopolymers. The melt viscosity of such asymmetric star polymers is generally significantly higher than their linear counterpart. Marrs et al (U.S. Pat. No. 3,658,740) discloses the preparation of PSA compositions by combining branched block copolymers with linear block copolymers, tackifiers and organic solvents. Marrs' PSA formulation requires a solvent as a critical element to provide an adhesive formulation which bonds to a wide variety of substrates but fails to address the need for hot melt processability. Nash (U.S. Pat. No. 4,163,764) discloses the preparation of PSA compositions employing a two-step process in which a monovinyl-arene monomer, such as styrene, is first polymerized, followed by a second stage where diene monomer and additional initiator are added and the resulting polymerized product linked to give linear or radially-branched polymers. These polymers, when formulated with tackifiers, exhibited superior tack and creep resistance. Feeney et al (U.S. Pat. No. 4,288,567) employs a branched block copolymer described in Prudence (U.S. Pat. No. 3,949,020) and relies upon a solution preparation process to achieve an adhesive composition having increased tack, faster molten solution time, and improved tack retention in hot melt blends.
While several references disclose the preparation of various copolymers which may be suited for use as a rubbery base material for PSA compositions, none known to applicants discloses the anionically-prepared copolymers containing organometallic-substituted styrene defined in the claims or the use of such copolymers in PSA compositions. The following discussion is intended to assist the reader in understanding related copolymer art.
According to Odian, Principles of Polymerization, 2nd Ed., Wiley-Interscience, p. 18, (1981) polymers fall into three structural groups: linear, branched and crosslinked. Branched polymer molecules are those in which there are side branches of linked monomer protruding from various central branch points along the main polymer chain and that have several idealized configurations. Branched polymers are known in at least three configurations. They may be "comb-like" where each branch is of equal length, "dendritic" where branches occur on branches (series branching), or "star-like" where all branches radiate from a single point.
Branching often imparts various desirable properties, for example, branched polymers have been made that have improved melt flow and processability. Additionally, appropriate branching disrupts long linear polymer backbones to thereby reduce crystallinity. In free radical and cationic polymerization processes, for example in the production of polyethylene, branching is largely uncontrolled and its extent is dependent on polymerization variables. In some cases branching can be as high as 15-30 branches per 500 monomer units. In contrast, anionic polymerization processes yield very narrow molecular weight distributions and a unique structure. Branched polymer structures produced by anionic polymerization are generally star shaped (arrayed about a central point or nucleus) although the structure can be varied by coupling together individually prepared arms of different structure.
Such polymers are described by St. Clair in U.S. Pat. No. 4,391,949 where "asymmetric" star block copolymers are prepared by mutually linking together individually prepared living polymers, which may be represented by (AB)Li and (C)Li, with polyalkenylaromatic linking reagents. The structural formula describing the resulting polymer is given as (A--B).sub.x --Y--(C).sub.z, where x plus z is greater than six. A statistical distribution of polymer products would be obtained from this process, where the average structure is equal to the mole ratio of the respective charges. Further chain growth would only be possible through the linking nucleus Y.
Crossland, U.S. Pat. No. 4,010,226, has also recognized the problem of preparing block polymers with an asymmetric configuration and, to avoid the statistical distribution of polymers obtained by St. Clair, first coupled a set of polymer arms with divinylbenzene, then continued the polymerization, utilizing the anionic centers that remain on the divinylbenzene residue, to produce a different set of arms bound to the same nucleus. The number of new arms grown would thus equal the number of arms coupled together, since linking with divinylbenzene (DVB) is a non-terminating process and each newly grown arm would have an anionic terminus. Fahrbach, U.S. Pat. No. 4,086,298, discloses star-block copolymers having a mixture of arms where some arms are formed by first polymerizing styrene with alkyllithium to form living polymer blocks, represented by (A)Li, and then adding a mixture of styrene and butadiene to form a graded copolymer represented by A-B.fwdarw.A' where the arrow represents a graded segment. Other arms are made up of only the butadiene-styrene graded copolymer segment. These arms are then linked together with a polyfunctional coupling agent, such as DVB, to give star-branched polymers. U.S. Pat. Nos. 4,221,884, 4,248,980, 4,248,982, 4,248,983, and 4,248,984, Bi and Milkovich, describe a similar series of polymers in which more complex polymer arm segments are linked together using a polyalkenyl aromatic, such as divinylbenzene, to form an asymmetric star molecule.
Prudence (U.S. Pat. No. 3,949,020) prepares branched block polymers by a method wherein divinylbenzene is added with the diolefin monomer to a polystyryllithium initiator. However, according to Bi and Fetters (Macromolecules 9, 732-742 [1976]), such a method leads to gelation when the divinylbenzene/initiator ratio is three or greater.
Martin, in U.S. Pat. Nos. 4,080,400, 4,143,089, 4,148,838, and 4,273,896, describes a composition obtained from the linking together of anionically active polymers (from, e.g., styrene) with silanes of the formula, X.sub.4-a-b Si(R).sub.b (CH.dbd.CH.sub.2).sub.a, where X is a displaceable group, R is alkyl, a is 1 to 4 and b is 1 to 3. One of the stated objects of these patents is to couple polymeric carbanions with silanes and then form new carbanions which can be used to initiate the polymerization of cyclic silicones or "other unsaturated monomers". No disclosure is provided directed towards the step of using other unsaturated monomers except for certain unspecified hydrocarbon/siloxane block polymers.
It has been established [Nametkin, Chemical Abstract Nos. 85:47314a (1976), 87:185046g (1977), and 89:110569n (1978)]that vinylsilanes of the type described by Martin will copolymerize in an anionic fashion, for example with butadiene; however, reactivity is very low, with up to 300 hours required for good conversion. Furthermore, copolymers of vinyl silanes with dienes initiated by butyl lithium are unimodal but exhibit peak broadening due to the occurrence of chain termination reactions caused by spontaneous cleavage reactions producing lithium hydride (Nametkin, Chemical Abstract No. 93:168679x, 1980). Loss of LiH during anionic homopolymerization of vinyltrimethylsilane has also been observed and has been used to explain the poor conversion and spread in molecular weight distribution observed in these polymers [Nametkin, Dokl. Nauk SSSR, 215, 861 (1974)]. Chaumont [Eur. Poly. J. 15, 537 (1979)]prepared vinylsilyl terminated polystyrenes via anionic polymerization; however, it was necessary to cap the polymer anion with diphenylethylene in order to reduce side reactions.
Chlorosilane-substituted styrenes are well-known compounds and have been used, for example, to prepare polysiloxane macromolecular monomers [Kawakami, Polymer J., 14, 913 (1982)]. Chromatography gels have been described based on poly-.alpha.-methylstyrene dianions and chlorodimethylsilylstyrene [Greber, Angew. Makromol. Chem. 1971, 16/17, 325]. Compositions for the encapsulation of electrical equipment have been derived from organosilicon monomers having styrenyl groups (Lewis, U.S. Pat. No. 2,982,757). Hirao et al. (Macromolecules 1987, 20, 242) has studied the anionic homopolymerization of (4-alkoxysilyl) styrenes and reaction of the resultant homopolymer with polystyryllithium.
There has been no disclosure, however, of the use of organometallic-substituted styrenes, e.g., chlorosilanesubstituted styrenes, in the preparation of condensed phase polymers or of PSA compositions made therewith.